The Tambu Lounge is very nice, but it does have some drawbacks. The bar itself is (unsurprisingly) themed as a Tiki bar which fits right in with our love of the whole island-living vibe. However, there aren't many seats at the bar and it's bang slap in the middle of the entrance to 'Ohana which means the lounge seating mainly serves as a waiting area for that. This is a problem common to quite a few onsite lounges and it doesn't really make for a civilised adult experience.
Mizner's is a bright, quiet, refined (although not stuffy) lounge where you'll find it easy to perch on a bar stool and strike up a conversation with the bartender or your fellow drinkers, if that's your thing.
Territory Lounge is all dark wood and subdued lighting. A very pleasant place to while away some time. Although it's right outside the entrance to Artist Point, it's self-contained so it doesn't seem to suffer from the "waiting room" phenomenon.
The Wave is modern, sophisticated and grown-up in a way that Disney doesn't usually do well. During early evening it's abuzz with the pre-dinner convention crowd. Well worth a trip from the Poly.
The Outer Rim isn't my idea of a great bar. It lacks something which I can't quite put my finger on. It seems like an afterthought.
The suggestion of asking about a seat at the bar at California Grill is a good one. If you're on your own, you shouldn't have any trouble at all. Just ask at the podium on level 2 of the Contemporary.